Metal lined composite risers in offshore applications

ABSTRACT

The present invention provides a metal lined composite riser section for use in offshore applications featuring a traplock metal-to-composite interface to secure a plurality of structural composite overwrap layers about a metal liner assembly. Each traplock is formed with at least one annular groove or channel which has been made in the exterior surface of the metal liner assembly. The annular trap grooves may be of various geometries and may be arranged adjacent to each other to form a traplock having 2 to 8 or any number of grooves required to ensure proper adhesion between the composite overwrap layers and the metal liner assembly. A number of structural composite overwrap layers are secured about the assembly by building up alternating combinations of helical and hoop fiber windings to form a composite material. The present invention also provides a method of making composite riser sections using either a mandrel or counterweights to facilitate the composite fiber winding process.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to commonly owned U.S. patentapplication Ser. No. ______, [Attorney Docket No. 1856-21200] entitled“Replaceable Liner For Metal Lined Composite Risers In OffshoreApplications” filed on the same date as the present application andincorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

[0003] Not applicable.

FIELD OF THE INVENTION

[0004] The present invention relates to metal lined composite risers andmethods of manufacturing composite riser assemblies of this type. Moreparticularly, the present invention relates to a metal lined compositeriser section featuring a metal-to-composite interface (MCI) having aplurality of structural composite overwrap layers attached to a metalliner assembly using traplock fittings.

BACKGROUND OF THE INVENTION

[0005] As offshore exploration and production of oil and gas has movedinto deeper water, it has become increasingly important to reduceweight, lower costs, and improve reliability of water-depth sensitivesystems such as risers and the like. The term riser generally describesvarious types of pipes or conduits that extend from the seabed towardthe surface of the water. By way of example only, these conduits may beused as drilling risers, production risers, workover risers, catenaryrisers, production tubing, choke and kill lines, and mud return lines.Conventional risers are normally constructed of various metal alloyssuch as titanium or steel. More recently, however, the oil and gasindustry has considered a variety of alternative riser materials andmanufacturing techniques including the use of composite materials.

[0006] Composite materials offer a unique set of physical propertiesincluding high specific strength and stiffness, resistance to corrosion,high thermal insulation, dampening of vibrations, and excellent fatigueperformance. By utilizing these and other inherent physicalcharacteristics of composite materials, it is believed that compositerisers may be used to lower system costs and increase reliability ofrisers used in deep water applications. Although there has been asignificant effort in the last decade to facilitate and to increase thegeneral use of composites in offshore applications, the acceptance ofcomposite materials by offshore operators continues to be a relativelyslow and gradual process. Reasonably good progress has been made toexpand the usage of composites for topside components such as vessels,piping and grating. Some advanced components such as high-pressure riseraccumulator bottles have already been used successfully in the field.However, in view of the reduced weight, extended life span, lower costand other enabling capabilities, composite risers are particularlyappealing for deep water drilling and production operations.

[0007] Composite risers are generally constructed of a series of jointsor sections each having an inner metal liner assembly and a number ofstructural composite overwrap layers which enclose the metal linerassembly. Typically, a metal liner assembly comprises a thin tubularmetal liner, usually of titanium or steel, coaxially secured to a metalconnector assembly. The connector assembly includes both ametal-to-composite interface (MCI) and a transition ring. The metalliner is secured to the MCI and the connector assembly through thetransition ring. The transition ring can be machined as an integral partof the connector assembly or made separately and then welded to theconnector assembly. The connector assembly is a standardized interfaceat the end of each riser section which facilitates the attachment of oneriser section to the next in series using flanges, threaded fasteners orthe like. The metal liner and the connector assemblies at each end arethen usually enclosed within an elastomeric shear ply, followed by acomposite overwrap reinforcement to form a composite riser section. Thecomposite riser section is then heated to cure the elastomeric shear plyand the composite overwrap. The elastomeric shear ply allows a smallamount of relative movement between the metal liner assembly and thecomposite overwrap to accommodate for differences in coefficients ofthermal expansion and elastic modulus. An external elastomeric jacketand a further composite overwrap may also be provided over the compositeriser section and thermally cured to provide additional impactprotection and abrasion resistance in an attempt to limit externaldamage to the composite riser section.

[0008] In application, the metal liner assembly functions to preventleakage due to the inherent cracking characteristics of the compositematerial itself. Over time, the matrix in the composite material willtend to develop micro cracks at pressures lower than those at which thecomposite fibers themselves will fail. The matrix micro cracking is dueto the thermal stresses induced by the curing cycle and the mechanicalstresses induced during the shop acceptance pressure test of thecomposite riser section during the manufacturing process. Thus, althoughthe metal liner assembly does not provide a great deal of mechanicalstrength to the riser, it functions to assure the fluid tightness of thecomposite riser and to prevent the leakage under conditions of matrixcracking which are inevitable.

[0009] The composite overwrap is secured to the metal liner assemblythrough the metal-to-composite composite interface (MCI). A traplock MCImay be used to mechanically lock a number of helical (axial) compositeplys into a series of annular grooves with several hoop(circumferential) plys of the composite forcing the helical plysdownward into the grooves. Accordingly, there is a need for a metallined composite riser which can offer the benefits of high strength andreduced weight, which has been designed to provide greater fieldreliability through the use of a traplock MCI that will ensure that thecomposite material remains firmly adhered to the metal liner assemblethroughout the useable lifetime of the riser.

SUMMARY OF THE INVENTION

[0010] The present invention provides a metal lined composite risersection for use in offshore applications featuring a traplock MCI tosecure a plurality of structural composite overwrap layers about themetal liner assembly. It is believed that a metal lined composite riserconstructed of sections according to the present invention will offeroutstanding strength to weight characteristics, durability and leakresistance and provide a useable lifetime which is comparable to that ofexisting titanium and steel risers used in offshore applications.

[0011] According to the present invention, a metal liner assembly of thecomposite riser section will be provided with a traplockmetal-to-composite interface (MCI) at each end. This traplock MCI may beincorporated into the connector assembly which is welded or attached toa commercially available metal liner through a transition ring. Eachtraplock is formed with at least one annular groove or channel which hasbeen made in the exterior surface of the metal liner assembly. As willbe discussed herein, these annular trap grooves may be of variousgeometries and may be arranged adjacent to each other to form a traplockhaving 2 to 8 grooves to ensure adequate load transfer capacity betweenthe composite overwrap and the metal liner assembly.

[0012] The metal lined composite riser section of the presentapplication typically comprises a metal liner assembly having a traplockMCI, an elastomeric shear ply disposed about the metal liner assembly,and a plurality of structural composite overwrap layers which aredisposed about the shear ply and held in place by the traplock MCI. Themetal liner assembly is formed of a metal liner as known in the art,such as carbon steel, stainless steel or titanium liner, and is usuallyfitted at each end with a connector assembly through a transition ring.The connector assemblies have a series of annular grooves which are cutinto the exterior surface of the assembly and disposed side-by-side toform a traplock. The transition rings are welded to the connectorassemblies to permit a smooth load transfer between the thin liner andthe thick connector assembly and to allow the use of different materialsfor the liner and the connector assembly. The connector assembliespermit sections of composite riser to be mated together, in series,using flanges, threaded fasteners or the like. The elastomeric shear plyis usually formed of a rubber like material, such as HydrogenatedAcrylonitrile Butadiene Rubber (HNBR), and completely covers the linerand the connector assemblies of the metal liner assembly. This shear plyis then further secured in place by at least one layer of hoop windingsof composite fiber which are placed at an angle almost perpendicular tothe longitudinal axis of the metal liner assembly. By way of exampleonly, suggested hoop windings may be wound at about plus or minus 80° tothe longitudinal axis of the assembly.

[0013] A number of structural composite overwrap layers are then securedabout the assembly to create a composite riser section according to thepresent invention. These overwrap layers may be built up of alternatinghelical and hoop fiber windings to form a composite material.Alternatively, a number of the helical windings may be supplemented,substituted or eliminated by sheets of prepreg composite material whichis then secured in place by the hoop windings. The helical windings orprepreg layers are intended to receive the axial loading of thecomposite riser section and to provide tensile strength in mostapplications. The hoop windings serve to provide resistance to hoopstresses induced by internal pressure and, of at least equal importance,also serve to secure the helical windings or prepreg plys and ensurethat they do not become detached from or slip relative to the metallining assembly.

[0014] The traplock MCI comprises at least one, and usually about 2 to8, grooves or traps which are cut about the circumference of the metalliner assembly near each end. By way of example only, a prepared metalliner assembly enclosed within a shear ply may be wound with a helicalply at plus or minus 10° relative to the longitudinal axis of the risersection. A substantially perpendicular hoop winding may then be placedabout the helical winding at plus or minus 80°. The hoop winding bindsthe helical winding and forces it downward into the groove of thetraplock. In one embodiment of the present invention, alternatinghelical and hoop layers or plys may be built up in pairs and groupedinto sets of three for each groove of the traplock MCI. Thus, by way ofexample only, a composite riser section constructed in accordance withthe present invention may comprise traplocks having six grooves at eachend of the metal liner assembly and the composite layers may be woundsuch that alternating helical and hoop layers are secured with the firstset of six layers held by the first groove of the traplock, closest tothe middle of the composite riser section, the next set of six layersheld by the second groove of the traplock, and so forth until the finalset of six layers are held by the sixth groove of the traplock and all36 layers (18 pairs) are firmly secured to the metal liner assembly toform a composite riser section. As noted earlier, it is also possible tosubstitute sheets of prepreg composite material by wrapping the liner in0° prepreg in place of the helical layers having plus and minus 10°windings. This 0° prepreg handles the axial loading of the compositeriser section, and is secured in place by the hoop windings much likethe helical windings which the prepreg is used in place of.

[0015] It should be understood that the traplock MCI may have any numberof grooves or traps as long as there is at least one proximate to eachend of the metal liner assembly. The number of traps and the totalnumber of structural composite overwrap layers may be varied dependingupon the actual loading conditions of the composite riser section andits intended end use application. Similarly, the wind angles of theoverwrap helical and hoop layers may be varied and the pattern in whichthey are laid up may also be changed so that a number of helicalwindings may be bound in place by a single hoop layer, rather thanalways alternating from helical to hoop in pairs. Again, the number ofhoop windings required need be only sufficient to withstand the hoopstresses applied to the composite riser section and to secure thehelical or axial load bearing layers about the metal liner assembly.Once the structural composite overwrap layers have been wound or laid upabout the metal liner assembly, it is necessary to apply heat to curethe composite material to complete the construction of the compositeriser section. It is also possible to further enclose this entirecomposite riser section within an additional elastomeric layer, such asHNBR, and apply non-structural composite overwraps to act as an externaljacket and to provide further protection from impact and abrasion damagewhich may occur as the riser section is put into use in the field.

[0016] It is further within the scope of the present invention toprovide a method of making composite riser sections of the type setforth and described herein. This may be done by providing a metal linerassembly having a traplock MCI and enclosing this metal liner assemblywithin an elastomeric shear ply. The metal liner assembly may then bemounted to a mandrel to support the liner assembly or counterweighted tofacilitate the composite fiber winding process by preventing theexcessive bending and torsional deformation in the thin metal liner dueto the weight of the uncured composite. The support system must alsoallow for the free rotation of the liner assembly during the filamentwinding and curing process to prevent sag in the composite laminatewhile still uncured. As noted earlier, the composite fibers aretypically applied in alternating pairs of helical and hoop plys, whichmay be gathered into sets of three and bound into the trap grooves ofthe MCI starting with the grooves closest to the middle of the risersection and working outwardly toward the ends of the riser section. As aresult, the first three pairs of composite layers are overwrapped by thesecond three pairs of composite layers and so forth until the finalthree pairs overwrap all of the previous ones. Thus, it is possible toapply a great deal of pressure, particularly to the innermost layers ofthe composite overwrap and hold them securely in place within the trapgrooves of the MCI. Following the winding or lay up steps, the entirecomposite wrapped metal liner assembly is placed into an oven and cured.Next, the elastomeric external jacket may be applied, an additional hoopwinding may be used to further secure the external jacket, andnon-structural composite plys may be added. Finally, the entire assemblyis placed into an oven and cured a second time to complete the compositeriser section according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be better understood in view of thedetailed description in conjunction with the following drawings in whichlike reference numbers refer to like parts in each of the figures, andin which:

[0018]FIG. 1 is an elevational view of a simplified schematicillustrating the use of risers in an offshore drilling and productionassembly;

[0019]FIG. 2 is a cross sectional view of a metal liner assembly for acomposite riser section constructed in accordance with the presentinvention;

[0020]FIG. 3 is a detailed drawing of a cross sectional view of thetraplock metal-to-composite interface portion of a metal lined compositeriser section constructed in accordance with the present invention;

[0021]FIG. 4 is a perspective view of a counterweight system for holdingthe composite riser during assembly without a mandrel; and

[0022]FIG. 5 is a detailed perspective view of a counterweight systemfor holding the composite riser during assembly without a mandrel.

DETAILED DESCRIPTION

[0023]FIG. 1 is a simplified schematic of a conventional offshoredrilling and production assembly [100] which illustrates the context ofthe present invention. An offshore platform [110] supports a derrick[120] which is a conventional apparatus for drilling or working over aborehole and producing hydrocarbons from the borehole. The offshoreplatform [110] is, in turn, further supported by pontoons [115]. Asubsea template [130] is provided on the seafloor [135] and a borehole[140] extends downwardly therefrom into the earth.

[0024] An elongated riser assembly [150] extends between the subseatemplate [130] and the platform [110], providing for fluid communicationtherebetween. The riser assembly [150] also generally comprises atieback connector [160] proximate to the subsea template [130] and anumber of riser sections [170] which extend between the platform [110]and the subsea template [130] and are connected thereto by a taper orflex joint [180] and a telescoping section [190]. The flex joint [180]and the telescoping section [190] are designed to accommodate themovement of platform [110] relative to the subsea template [130] and theborehole [140]. The composite riser joints or sections [170] thatcomprise the riser assembly [150] are coaxially secured together bythreaded fasteners or other mechanical fastening devices, as known inthe art. Each riser section [170] must accommodate the pressure of thefluid or gas within the section, the tensile load which is caused bysuspension of additional riser sections below that section, and thetensile loads and bending movements which are imposed by the relativemotion of the platform [110] with respect to the subsea template [130].

[0025] In a composite riser section according to the present invention,metal connectors are coaxially secured to metal liner to form a metalliner assembly which is subsequently wrapped with an elastomeric shearply, a number of structural composite overwrap layers, and an externalelastomeric jacket providing additional impact and abrasion resistance.The composite overwrap further comprises a number of individual layerswhich are applied about the metal liner assembly at various anglesrelative to the longitudinal axis of the composite riser section. Eachof these layer or plys are wound or applied one at a time and consist ofa number of small diameter fibers (e.g., from about 6 to about 10microns) having high specific strength and modulus properties which areembedded in a polymer matrix material.

[0026] The polymer matrix material, usually some form of resin or glue,has bonded interfaces which capture the desirable physicalcharacteristics of both the embedded fibers and the matrix itself. Inshort, the fibers carry the main loads which may be applied to thecomposite material while the matrix maintains the fibers in thepreferred orientation. The matrix also acts to transfer loads acrosslarge numbers of fibers and to protect the fibers from the surroundingenvironment. The resulting composite material properties depend uponboth major components, the fibers and the polymeric matrix. A number ofknown thermosetting or thermoplastic polymeric matrixes may be used toproduce the composite riser section in accordance with the presentinvention. For the composite riser section of the present invention,preferred matrix materials may include various vinyl esters and epoxieswith glass transition temperatures above about 270° F. By way ofexample, one preferred resin is EPON 862 (available from ResolutionPerformance Products of Houston, Tex.) amine-cured epoxy formulated withan additional hardener and curing agent. Although it is not a structuralconcern, the components of the resin are preferably selected to avoidsuspected carcinogenic compounds, particularly MDA curing agents.

[0027] A number of fiber types may be used for forming suitable overwraplayers on the composite riser section. Fibers are usually gradedaccording to the tensile modulus as measured in millions of pounds persquare inch (msi). One type of preferred fiber is a low cost, mediummodulus (about 32 msi to about 44 msi, and preferably 35 msi)polyacrylinitrile (PAN) carbon fiber. Several acceptable fibers of thistype are HEXEL AS4D-GP, available from Hexel Corp. of Stamford, Conn.;GRAFIL 34-700, available from Grafil of Sacramento, Calif.; and TORAYT700SC (LMS-R10544), available from Toray of Tokyo, Japan. Another typeof preferred fiber is a high modulus (about 55 to about 80 msi, andpreferably about 55 msi) PAN carbon fiber in either tow form or uniaxialprepreg mats. Acceptable grades of this fiber include PYROFIL 56-700,available from Grafil; and TORAY M40J, available from Toray.Alternatively, a hybrid of glass and carbon fibers incorporated into thematrix material may also provide acceptable results. One preferred formof glass fiber is commonly known as E-glass fiber, availablecommercially as PPG 1062-430, available form PPG of Pittsburgh, Pa.; andOWENS-CORNING 30-158B-450, available form Owens-Corning of Toledo, Ohio.

[0028]FIG. 2 shows a metal liner assembly [200] suitable formanufacturing a composite riser section which comprises a tubular liner[210] and connector assemblies [220] attached at opposite ends. Thetubular liner [210] may be formed of titanium, steel, or other metalalloys suitable for offshore oil and gas production applications. Insome instances, it may be desirable to incorporate additional corrosionresistance by using a stainless steel liner [210].

[0029] Still referring to FIG. 2, in accordance with the presentinvention, the connector assembly [220] of the composite riser sectionfeatures a traplock MCI [240] and is welded or affixed to a transitionring [260] located between the MCI [240] and the liner [210]. The MCI[240] further comprises a number of trap grooves [250] for securing astructural composite overwrap, not shown here. As shown here, themechanical connector [270] is usually formed of titanium, steel or thelike and is welded to the connector assembly [220] to provide a numberof fittings for mechanically fastening the composite riser sectionstogether in series to form a riser assembly between the seafloor and theproduction platform. It should be understood that although the metalliner assembly [200] shown in FIG. 2 is formed of at least sevenseparate components (i.e., a liner [210], two transition rings [260],two MCIs [240], and two mechanical connectors [270]) which aresubsequently welded together to form a single assembly [200], it wouldbe possible to fabricate a metal liner assembly from three tubularsections (i.e., a liner [210] and two connector assemblies [220] thateach include a transition ring, an MCI and a flange machined from asingle piece of tubing) to create a metal liner assembly [200].

[0030]FIG. 3 shows a detailed partial cutaway of a composite risersection [400] constructed in accordance with the present invention. Notethat each connector assembly [220] further comprises a traplock MCI[240] having a plurality of outer grooves [250] which are shown here.Although a series of six trap grooves [250] are shown disposedsided-by-side, the number of grooves can vary as appropriate for theintended end use of the riser section [400]. Additionally, the trapgrooves [250] may take a number of different configurations in that theymay be cut at about 90° to the surface of the metal liner assembly [200]and may have sidewalls which are cut at right angles to form a squarechannel or alternatively may have sidewalls which are angled inward toform a trapezoidal groove. The sidewall angle of the grooves normallyranges from about 30 to about 60 degrees and may differ on opposingsidewalls. The trap grooves [250] may also be cut to different depths tocreate a stepped arrangement, as shown. Regardless of the geometry, eachgroove [250] acts as a mechanical interlock joint which is fabricatedinto the outer surface of the MCI [240].

[0031] An elastomeric shear ply [300] in an uncured state is typicallyapplied to the outer surface of the metal liner assembly [200] of FIG. 2to provide for a high shear strain capacity to accommodate small amountsof movement between the composite overwrap [350] and the metal linerassembly [200] because of differences in the thermal expansioncoefficient and the elastic modulus. One preferred elastomeric shear ply[300] is formed of Hydrogenated Acrylonitrile Butadiene Rubber (HNBR)and is bonded using CHEMLOK 205 primer and CHEMLOK 238 adhesive to theliner and part of the connection assemblies [220] outboard of thetraplock MCI [240]. The elastomeric shear ply [300] can have anysuitable thickness, and the thickness can vary at particular regions ofthe metal liner assembly [200] to achieve desired characteristics. Byway of example only, the thickness of one preferred elastomeric shearply [300] may be about 0.09 inches over the entire length of the linerportion [210] of the metal liner assembly [200], while the shear ply[300] thickness may be reduced to about 0.01 inches over the grooves[250] of the traplock MCI [240]. The reduced thickness of the shear ply[300] in the grooves [250] allows the bearing surfaces in the traplockjoint to move without damage to the structural composite overwrap [350]and improves the bearing performance of the composite riser section[400].

[0032] The structural composite overwrap [350] is a composite tubecomprising carbon, glass or other reinforcing fibers embedded in anepoxy matrix, as previously described herein, which is fabricated overthe metal liner assembly [200] using built-up layers via a filamentwinding process. Generally, the composite overwrap [350] is wound overthe elastomeric shear ply [300] which has been applied to metal linerassembly [200], as described hereinabove. The composite overwrap [350]includes helical layers that extend in a generally axial direction alongthe metal liner assembly [200] from end to end and hoop layers that areapplied substantially perpendicular to the helical layers about thecircumference of the metal liner assembly [200]. The helical fiberlayers of the composite and the elastomeric shear ply [300] arecompacted into the trap grooves [250] of the MCI [240] and are heldsecurely in place by the hoop windings of the composite overwrap [350].

[0033] The filament winding process for fabricating the compositeoverwrap [350] over the metal liner assembly [200] is generallydescribed as follows. Composite overwrap [350] comprises alternatinghelical and hoop layers of fiber, including an initial consolidatinghoop layer which is wound over the elastomeric shear ply [300]. Afterwinding each of the fiber and matrix helical layers, the helical layeris compacted into a trap groove [250] with hoop windings. In thismanner, a number of subsequent helical layers are also compacted intoeach of the trap grooves [250]. Localized reinforcing layers of fiberand matrix, preferably a prepreg material, may also be applied over MCI[240] and compacted into each of the trap grooves [250] to improve theload sharing across the grooves [250] and to increase the strength ofthe MCI [240]. By way of example only, the thickness of the individualhelical and hoop fiber layers may be about 0.015 inches to about 0.050inches. A final layer of hoop windings is wound over the entire lengthof the metal liner assembly [200], including MCI [240], therebycompleting the filament winding of composite overwrap [350]. Otherfilament winding processes recognized in the art may be suitable forfabricating the composite riser section of the present invention.

[0034] Various strength characteristics and other mechanical propertiesof the composite riser section [400] may be adjusted by varying the windangle of the composite overwrap [350]. It is possible to make usefulriser sections having helical or axial load bearing plys ranging fromabout 0° to about plus or minus 20° to the longitudinal axis of theriser section. Likewise, the hoop plys should generally lay nearlyperpendicular to the underlying helical ply and range from about 90° toabout plus or minus 70° to the longitudinal axis of the riser section[400]. Using conventional fiber winding techniques, however, it ispreferable to have a helical winding angle of at least about plus orminus 5° and corresponding hoop winding angle of not less than aboutplus or minus 85°. In place of one or more helical or hoop layers, 0°prepreg plys may be laid-up at 0° and 90°, but these plys will requireadditional hoop windings to compress the prepreg into the MCI [240] andensure that is conforms to the metal liner assembly [210].

[0035] By way of example only, one preferred embodiment of the presentinvention is a composite riser section having 6 MCI trap grooves at eachend and 36 total layers of structural windings about the metal riserassembly. For this riser section, a Grade 9 titanium liner assembly isprepared with a HNBR shear ply and a 55 msi hoop winding to form aninitial consolidating hoop layer across the entire length. A carbonfiber (33 msi) helical layer is then applied at a 10° wind anglefollowed by a hybrid (55 msi) hoop layer at −80°. The next pair ofstructural plys is applied at −10° and 80°, the following pair isapplied at 10° and −80°, and so forth. For every three pairs of windingsa new MCI trap groove is started, working from the innermost groove,nearest the middle of the riser section, outward until all six traps arefilled.

[0036] Optionally, it is possible to further reinforce the traplock[240] region and spread the applied loads more evenly across all of thetrap grooves [250] by further incorporating a 0° carbon prepeg (55 msi)layer after the second and third helical winding layer for each trapgroove [250]. Similar riser sections [400] may be produced having atleast 1 trap groove [250] at each end of the riser section [400] withabout 1 or more pairs of windings per groove [250]. It may also bedesirable or cost effective to apply two or more helical windings to theliner [200] before each hoop winding. Obviously, a nearly infinitenumber of winding variations may be used as limited only the designer'simagination and the structural loading requirements of a particularapplication.

[0037] After the filament winding is complete, the wound assembly istransferred to an oven, not shown, or heating elements are placed aboutthe composite assembly where heat is applied to cure the thermosettingmatrix of the composite overwrap [350] and the elastomeric shear ply[300]. After curing, an external jacket [450] of uncured elastomericmaterial, such as HNBR, may be applied over the entire length of theresulting composite riser section [400] to prevent migration of seawaterinto the composite wall and through its interface with the MCI [240].The external elastomeric jacket [450] provides additional impactprotection, mitigating possible damage caused by dropped tools ormishandling of the composite riser section [400]. An additionalcomposite layer [500] of E-glass or other reinforcing fibers such ascarbon in a polymeric matrix may be filament wound over the externalelastomeric jacket [450] to further compact the jacket [450] during thecure and to provide additional scuff protection. The composite risersection [400] is then heated a second time to a suitable temperature tocure the elastomeric external jacket [450] and composite outerwrap[500].

[0038] In the manufacture of a composite riser section the metal linerassembly must be held in a horizontal position but allowed to rotateabout its axis to facilitate fiber spinning. Of course the compositeoverwrap layers have a significant amount of weight, particularly duringfiber spinning in which the matrix resin material is still wet. Thecomposite overwrap will cause an unsupported metal liner assembly toflex or bow in the middle during manufacture. This would result in avery poorly constructed composite riser section that would almostcertainly be too curved for use. Composite risers are generallyconstructed using a steel mandrel which is inserted trough the linerassembly to support the weight of the composite overwrap during thefiber winding process. After the composite overwrap has cured, themandrel is removed and the riser section is ready for use. However,because the metal liner is normally very thin walled, usually 2-4 mmthick, and because the composite overwrap will tend to bow the risersection in the middle, the process of removing the steel mandrel fromthe completed riser section may cut or gouge the metal liner. In somecases, the metal liner is so badly damaged that a new composite risersection must be scrapped entirely. Accordingly, a method of makingcomposite riser sections without inserting a mandrel would be desirableand could significantly improve manufacturing efficiency by reducingscrapped parts.

[0039] Referring now to FIG. 4, a perspective drawing of a counterweightsystem for use in assembling composite risers without a mandrel isshown. The composite riser is constructed by holding a metal linerassembly in a horizontal position and then winding fiber about theexterior surface. As shown here, a metal liner assembly [200] is held ina horizontal position between two supports [600] having a number ofrollers [610] which permit the liner assembly [200] to rotate freelyabout its longitudinal axis. The liner assembly [200] is further securedby two short mandrels or plugs [620] inserted into the bore of the linerassembly [200] at opposite ends. The plugs [620] have an outer diameterthat is slightly less than the inner diameter of the liner assembly[200] and are designed to extend into connection assembly [220] but notinto the liner [210] itself. The plugs [620] may also extend outwardfrom the liner assembly [200] to create leverage by interaction with therollers [610] of the supports [600].

[0040] During the manufacturing process, the liner assembly [200] isfitted with a plug [620] at each end and placed in the supports [600].The rollers are then clamped into place about the connection assembly[220] and the extended portion of the plugs [620] to ensure that theonly movement permitted is about the longitudinal axis. The supports[600] are set at a distance apart that is slightly less than the totalrelaxed length of the liner assembly [200]. Thus, the liner assembly[200] should be bowed slightly upward in the middle prior to winding thecomposite overwrap, not shown. The supports must be weightedsufficiently to hold the liner in this bowed condition. As the compositematerial is applied to the liner assembly [200], the weight of thecomposite will exert force upon the liner assembly [200] and cause it tostraighten out or flatten in the middle. The liner assembly [200] mustbe checked during fiber winding to ensure that it is not permitted tosag. The supports [600] may be pushed slightly closer together duringthis process, if needed.

[0041] As best seen in FIG. 5, a detailed perspective drawing of thecounterweight system illustrates the manner in which a composite riseris held horizontally and rotated to facilitate fiber spinning. As shownhere, the support [600] is constructed of steel plates or angles, butcould be manufactured of other materials and then weighted to avoidmovement during fiber winding. It is also shown that a first pair ofrollers [610] is in contact with the connection assembly [220] and thata second pair of rollers is in contact with the plug [620]. As notedearlier, the rollers [610] hold the liner assembly [200] securely inplace to prevent lateral movement but allow the liner assembly [200] torotate about its longitudinal axis. Of course, while two pairs ofrollers [610] are shown here for supporting the liner assembly [200], itis understood that a variety of roller arrangements may be used and theexact number or positioning may be altered as long as the liner assembly[200] is not free to move laterally, is free to rotate about its axis,and is bowed slightly upward in the middle prior to fiber winding.

[0042] The use of metal lined composite risers should providesignificant benefits once these risers are produced on a commercialscale. Preliminary investigations and cost analysis has revealed thatcomposite risers constructed in accordance with the present inventionoffer reduced weight, improved vibration dampening, improved thermalinsulation, and substantial cost savings. In regard to weight, for atypical 22 inch diameter riser section, the metal lined composite risersection should be about ⅔ of the weight of a titanium riser section andabout ½ of the weight of a steel riser section. Concerning fabrication,the composite riser section should cost about 1-1½ times the cost of asteel riser section and only ½ the cost of a titanium riser section.Although the composite riser section will cost a bit more than the steelriser section, it is important to note that it usually costs about $4-7per pound of topside weight on an offshore facility. By decreasing theweight of the riser section to ½ that of steel, the additionalfabrication cost will be more than offset. Moreover, the reduced weightof the composite riser section will make it easier to handle and requireless power to move thereby reducing wear and tear on the existingdrilling platform machinery.

[0043] As noted earlier, composite risers also offer improved thermalinsulation. This too is of greater importance as water depth increases.Many conventional risers require heating to maintain the desired fluidviscosities within the riser. This may be both difficult and somewhatexpensive. By way of comparison, the thermal conductivity of a typicalsteel riser may be compared to that of a composite riser. Water has athermal conductivity of about 0.6 W/m-C, a steel riser has a thermalconductivity of about 50 W/m-C, and a composite riser has a thermalconductivity of about 0.5 W/m-C. As might be expected, the steel riserhas a very high thermal conductivity and transfers heat from inside theriser to the surrounding seawater at a very high rate. In contrast, thecomposite riser almost matches the thermal conductivity of thesurrounding water. Moreover, if heating were required, heating elementscould be incorporated into the composite overwrap layers duringfabrication of the composite riser.

[0044] Another property of composite risers is improved dampeningcharacteristics. If exploited fully in drilling risers, the dampeningcharacteristics may reduce or eliminate the need for strakes commonlyused to suppress vortex induced vibrations. Preliminary test data hasindicated that composite risers offer a structural dampening which isnearly equivalent in value to conventional hydrodynamic dampening.Additionally, higher dampening composite risers may be produced bytailoring the laminate structure, i.e. introducing interleaf layers, tomaximize this particular property.

[0045] Additional background information regarding composite drillingrisers is disclosed in each of the following articles which areincorporated by reference herein in their entirety: Composite Risers areReady for Field Applications—Status of Technology, Field Demonstrationand Life Cycle Economics, 13^(th) International Deep Offshore TechnologyConference (DOT 2001), Rio de Janeiro, Brazil, Oct. 17-19, 2001:Remaining Challenges of Advanced Composites for water depth sensitivesystems, presented at the 2^(nd) Annual Deep Offshore Technology Int.Conf. Held in New Orleans, La. on Nov. 7-9, 2000; OTC 11006: DesignConsideration for Composite Drilling Riser, presented at the OffshoreTechnology Conference held in Houston, Tex. on May 3-6, 1999; SPE 50971:Composite Production Riser Testing and Qualification, SPE Production &Facilities, August 1998 (p. 168-178). These papers also present aconsiderable amount of economic cost data for comparison of variouscomposite structures for offshore applications relative to conventionalsteel ones.

[0046] While a preferred embodiment of the invention has been shown anddescribed herein, modifications thereof may be made by one skilled inthe art without departing from the spirit and the teachings of theinvention. The embodiments described herein are exemplary only, and arenot intended to be limiting. Many variations, combinations, andmodifications of the invention disclosed herein are possible and arewithin the scope of the invention. Accordingly, the scope of protectionis not limited by the description set out above, but is defined by theclaims which follow, that scope including all equivalence of the subjectmatter of the claims.

What we claim as our invention is:
 1. A composite riser section foroffshore applications comprising: a metal liner assembly including atraplock MCI having at least one trap groove on an outer surfaceproximate to each end of the metal liner assembly; a plurality ofstructural composite overwrap layers disposed about the metal linerassembly; and wherein the plurality of structural composite overwraplayers further composes alternating helical plys and hoop plys.
 2. Thecomposite riser section of claim 1 wherein at least one helical ply is a0° prepreg ply.
 3. The composite riser section of claim 1 wherein eachhelical ply is compressed by an overlying hoop ply to fit snugly intothe at least one trap groove at each end of the metal liner assembly. 4.The composite riser section of claim 2 wherein each trap groove holds 3pairs of alternating helical and hoop plys.
 5. The composite risersection of claim 1 wherein the at least one trap groove is a trapezoidalchannel.
 6. The composite riser section of claim 1 wherein the at leastone trap groove is a rectangular channel.
 7. The composite riser sectionof claim 1 wherein the traplock MCI has at least 3 trap grooves on anouter surface proximate to each end of the metal liner assembly.
 8. Thecomposite riser section of claim 1 wherein the metal liner assembly isformed of metal selected from the group consisting of titanium, steel,stainless steel and combinations thereof.
 9. The composite riser sectionof claim 1 wherein the metal liner assembly further comprises a weldedtransition ring.
 10. A method of making a composite riser section foroffshore applications comprising the steps of: providing a metal linerassembly including a traplock MCI having at least one trap groove on anouter surface proximate to each end of the metal liner assembly; andwinding resin impregnated fibers about the metal liner assembly to forma structural composite overwrap; and wherein the step of winding resinimpregnated fibers about the metal liner assembly further comprises thestep of winding helical plys and hoop plys in an alternating manner. 11.The method of claim 10 wherein the step of winding helical plys and hoopplys in an alternating manner further comprises the step of compressingeach helical ply into the traplock MCI with an overlying hoop ply. 12.The method of claim 11 wherein the traplock MCI has at least one trapgroove and the step of compressing each helical ply further comprisesfilling each trap groove with at least three pairs of alternatinghelical and hoop plys.
 13. The method of claim 10 further comprising thestep of laying-up strips of uncured rubber material to form anelastomeric shear ply about the metal liner assembly prior to windingresin impregnated fibers.
 14. The method of claim 13 further comprisingthe step of coating the traplock MCI with mold release prior tolaying-up strips of uncured rubber to form an elastomeric shear ply. 15.The method of claim 13 further comprising the step of applyingsufficient heat to cure the elastomeric shear ply and the structuralcomposite overwrap.
 16. The method of claim 15 wherein the step ofapplying sufficient heat to cure the elastomeric shear ply and thestructural composite overwrap further comprises the steps of holding thepart at a temperature of about 150° F. to about 175° F. for from about12 to about 13 hours, and then holding the part at a temperature ofabout 290° F. to about 310° F. for from about 8 to about 9 hours. 17.The method of claim 15 further comprising the steps of: laying-up stripsof uncured rubber material to completely enclose the structuralcomposite overwrap in an external jacket; winding resin impregnatedfibers over the external jacket to form a scuff-resistant protectivelayer; and applying sufficient heat to cure the external jacket and thescuff-resistant protective layer.
 18. The method of claim 17 wherein thestep of applying sufficient heat to cure the external jacket and thescuff-resistant protective layer further comprises the step of holdingthe part at a temperature of about 290° F. to about 310° F. for fromabout 4.5 to about 5.5 hours.
 19. The method of claim 10 furthercomprising the step of mounting the metal liner assembly on a mandrelprior to winding resin impregnated fibers.
 20. The method of claim 10further comprising the step of holding the metal liner assembly betweenat least two counterweighted supports to minimize deflection prior towinding resin impregnated fibers.
 21. The method of claim 20 furthercomprising the step of positioning the at least two counterweightedsupports such that the metal liner assembly is bowed slightly upwardprior to winding resin impregnated fibers.
 22. The method of claim 20wherein the counter weighted supports further comprises rollers topermit rotation of the metal liner assembly during the winding resinimpregnated fibers.
 23. The method of claim 10 further comprising thestep of bowing the metal liner assembly slightly upward prior to windingresin impregnated fibers.
 24. The method of claim 10 further comprisingthe step of laying up at least one 0° prepreg ply during the step ofwinding resin impregnated fibers.
 25. The method of claim 12 furthercomprising the step of laying up a 0° prepreg ply following the secondand third helical plys and prior to winding the second and third hoopplys during the step of winding resin impregnated fibers.
 26. The methodof claim 10 further comprising the step of laying up at least one 90°prepreg ply during the step of winding resin impregnated fibers.
 27. Acomposite riser section manufactured according to the method of claim10.